<!DOCTYPE html>
<html>
<head>
 <style>
  a:link,
  a:visited {
   background-color: white;
   color: black;
   border: 2px solid green;
   padding: 10px 20px;
   text-align: center;
   text-decoration: none;
   display: inline-block;
  }

  a:hover,
  a:active {
   background-color: green;
   color: white;
  }
 </style>
</head>
<body>
<a href="javascript:alert('Are you OK?');">点我</a>
</body>
</html>
